1. Field of the Invention
This patent relates to a method for measuring the characteristic angles of the attitude of a vehicle for the purpose of its regulation, and the device for implementing the method.
2. Description of Background Art
Periodically checking the attitude setting of a vehicle is important in ensuring the best road holding and best driving comfort.
Road holding is directly proportional to the vehicle road adherence, which is itself a function mainly of two factors, namely the area of contact of the tires with the ground, and the wheel drift, these both depending on the geometry of the vehicle frame and of the suspensions.
The geometry of a vehicle frame provided with wheels and suspensions defines the characteristic wheel angles, ie the angles between each wheel and the others, between each wheel and the longitudinal and transverse frame axes, between each wheel and the vehicle running axis and between each wheel and the vertical to the ground, and the steering angles. The correct values of these angles are dictated by the vehicle manufacturer.
These angles must therefore be able to be measured in order to set them to the appropriate values.
The characteristic angles are:
left, right and total front convergence
left, right and total rear convergence
right and left front camber
right and left rear camber
right and left incidence
right and left kingpin
front and rear set-back
thrust angle
track difference
The text will be better understood from the following definitions of the characteristic angles:
the convergence is the angle formed between the plane perpendicular to the axis oil each wheel and the longitudinal axis of symmetry of the vehicle;
the total convergence is the sum of the angles of convergence of the wheels pertaining to one and the same axle;
the track angle is the angle formed between the plane perpendicular to the axis of each wheel and the vertical plane;
the angle of incidence is the angle between the projection of the steering axis onto the vehicle longitudinal plane and the vertical;
the kingpin angle is the angle formed between the projection of the steering axis onto the vehicle transverse plane and the vertical;
the set-back is the misalignment of the wheels of one and the same axle to the vehicle axis of symmetry;
the thrust axis is the axis defined by the bisector of the total rear convergence angle;
the track difference is the angle between the line joining the wheels positioned on one and the same side, but pertaining to two different axles, and the vehicle axis of symmetry.
The most recent known devices used for regulating the attitude of vehicle wheels are based on opto-electronic measuring and sensing instruments.
These devices comprise angle measuring instruments which are generally applied to the vehicle wheels and mutually interact mechanically or optically, or are fixed to the ground to obtain the image of reference markers positioned on the wheels. These reference markers can be either solid locators fixed to the wheels, or images projected onto the wheels.
The data obtained by said devices are fed to a processor which processes them using known geometrical formulas which give as their result the values of the characteristic angles of the frame attitude.
Known devices however suffer from a series of problems which substantially limit both their use and their accuracy.
A first important drawback is the time required for taking the measurements and making the relative adjustment. In this respect, in known devices, more than twenty operations are required for preparing the vehicle and taking a measurement, and it has been found that the preliminary operations involve on an average more than 70% of the time required for carrying out a normal measurement cycle.
A further drawback is determined by the systematic errors introduced by the compensation operation.
The compensation operation reduces the measurement errors deriving from the geometrical deformation of the wheel rim and from the sensor-wheel coupling.
This is achieved by introducing into the measurement a correction factor calculated using data obtained during one turn of the vehicle wheel.
However to achieve a wheel turn without involving considerable space the wheel has to be raised from the ground, so releasing the vehicle weight from the suspension-shock absorber unit.
However this introduces errors due to the fact that during their working stroke the suspensions induce variations in the characteristic angles of the vehicle, hence raising the wheel from the ground leads to the drawback of finding the wheels positioned in a different manner from that when in the running state, which is achieved only after the suspensions have settled.
A further limit of current aligners is the maximum range of angular measurement of the transducers. In particular, this limit is apparent in measurements on the steering geometry.
Motor vehicle manufacturers have generally used a linkage scheme for the steering members known as a xe2x80x9cJeantaud trianglexe2x80x9d. This configuration ensures concentricity of the circumferences travelled by the wheel through large-radius bends, however for steering through a smaller radius of curvature the Jeantaud triangle creates a condition which progressively deviates from the ideal.
It is therefore apparent how important it is to make accurate measurements on the steering geometry in order to guarantee the safety and road holding of the vehicle.
For effecting these measurements, currently available opto-electronic aligners use rotating plates positioned below the steered wheels and provided with electronic devices. The steering angle is measured by graduated pointers or by electronic transducers which measure the plate rotation.
These systems partly overcome the limited measurement range of transducers, but make indirect measurements which are subject to errors due to the roto-translational movement of the plate. In this respect, during steering, the wheel movement can be resolved into a translation component and a rotation component, this phenomenon being due to the steering axis not passing through the wheel center.
It is apparent that an indirect measurement, such as that made by known devices, is subject to errors due to the double nature of the phenomenon to be measured.
The attitude can be measured and then regulated either by apparatus which take the measurements using devices connected to the wheel rim or by apparatus which do not use any device fixed to the wheel rim.
Apparatus are known for measuring the characteristic attitude angles by mounting measuring devices on all the vehicle wheel rims.
These devices are in the form of goniometers or angle transducers interacting with identical instruments fixed to the wheel rims of two adjacent wheels, in the transverse and longitudinal directions.
This interaction between said instruments can be achieved either mechanically by means of wires or springs, or by electrical and/or optical devices.
When the measurements have been taken, a processor processes the measured data by mathematical algorithms of known type.
Devices are also known which use measuring systems in which no reference material is positioned on the wheel rim.
U.S. Pat. No. 4,899,218 in the name of WALDECKER illustrates a measurement process based on projecting an oscillating structured light beam onto the wheel in such a manner as to produce at least two reference images on it.
Said images are read by video cameras positioned at a certain angle to the optical plane defined by the plane of oscillation of said beam.
The spatial position of the outlines and hence of the wheel is calculated by a computer using a known triangulation system.
Having defined the spatial wheel position in this manner, a processor calculates the characteristic attitude angles relative to a suitable reference system.
The devices utilized in the stated patent are very complicated to use, especially with regard to the procedure for calibrating the apparatus.
Published patent application DE 2948573 in the name of SIEMENS describes an apparatus in which the measurement process is based on pairs of images of the wheel rim edge or of a circumference thereof, taken from different angles.
From these images, ellipses are obtained, from the parameters of which the spatial position of the wheel can be determined knowing the exact position and orientation of the video cameras.
The publication gives no indication of how the system is able to distinguish the edge of the wheel rim, or a concentric circumference positioned thereon, from the tire which surrounds it, and also does not teach how to take account of the inevitable distortions of the ellipse image representing the wheel rim edge, which obviously negatively influence the accuracy of the measurement.
In this respect, the image seen by the video camera is presented as a distorted ellipse in that the side closer to the video camera appears wider than the more distant side and consequently, in order not to commit measurement errors, account should also be taken of the distortions.
In conclusion, all the known devices are affected by systematic measurement errors due to the inevitable inaccuracies caused by the operator in positioning the measuring devices on the wheel rims, or during the compensation operations which require the vehicle wheels to be raised and each wheel turned, or by errors from the said optical distortions, or by irregularities in the shape of the type on which the images are produced, or of the wheel rim itself.
The object of this patent is to eliminate the aforesaid drawbacks by providing a method and measuring device able to operate without the need to apply specific reference markers or sensors to the wheel, not even in the form of a projected image, and which enables the compensation operation to be carried out without raising the vehicle wheels from the ground, while substantially reducing the time involved in the preparation for and in the execution of the measurement of the characteristic attitude angles.
The method by which the apparatus executes the measurement comprises obtaining and processing a certain number of pairs of images of each wheel while the still moving vehicle is being positioned on the measurement site, for the purpose of identifying the spatial position of the wheel rim. This is achieved by seeking within the images of the circular structures, and identifying, those regions in which the transition between the image grey levels is a maximum, as will be clarified hereinafter.
In this manner, points are determined pertaining to segments of the wheel rim edge, which are processed and associated with a Cartesian reference system, to accurately determine its spatial position by mathematical processes of known type to which reference will be made hereinafter.
Having determined a certain number of points pertaining to the wheel rim edge, the invention calculates for each image the ellipse passing through these points.
Alternatively, the ellipse is calculated as the interpolation of a discrete but large number of points lying on the image of the wheel.
Having determined two ellipses for each wheel from images taken from two positions at different angles, the invention establishes a correspondence between the edge elements of the two ellipses.
In this manner, the edge of the wheel rim and the plane containing it, and hence the wheel track and convergence of each individual wheel are unequivocally determined.
It should be noted that the determination of the ellipses is not essential for the purpose of identifying the edge of the wheel rim and the plane containing it, but is done in that it enables the data to be more rapidly processed as it concentrates the search for the points representing the edge of the wheel rim into limited zones.
The invention achieves this result by a method for measuring the vehicle attitude comprising two video cameras for each wheel having their lens facing the measurement site and connected stably thereto. The video cameras are connected by known means to a processor arranged to process the images of each wheel as seen by them, in order to determine the wobble of the wheel rim, ie the non-perpendicularity of the plane of the wheel rim to the axis of rotation thereof, and to calculate the characteristic attitude angles.
For each wheel the two video cameras obtain several pairs of images with the wheel in movement, the invention being able to determine the wobble of the wheel rim about the axis of rotation by known algorithms.
By comparing data relating to the angular variation in the horizontal, vertical and rotational planes measured in at least two different steered positions, the invention also calculates the spatial position of the steering axis and is hence able to calculate all the characteristic angles of the vehicle attitude and to display them on the processor monitor to enable the operator to make the necessary adjustments.
It is apparent that the invention can achieve the same result using only one video camera per wheel or only one video camera in total with the consequent use of suitable optical paths, formed with the aid of fixed and/or movable mirrors and possible shutters, which enable all the wheels to be seen in succession from the point of observation.
The particular characteristics of the device and of the method are specified in the claims.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.